1. Field of the Disclosure
The present disclosure relates to a communication method for wirelessly transmitting a signal which has been modulated by an OFDM (Orthogonal Frequency Division Multiplexing) method or an SC-FDMA (Single-Carrier Frequency-Division Multiple Access) method and a transmission apparatus employing the communication method.
2. Description of Related Art
Hitherto, high-speed communication employing a modulation method using OFDM such as LTE (Long Term Evolution) or WiMAX (Worldwide Interoperability for Microwave Access) has been put into practical use. OFDM has a guard interval, and therefore, has ability to particularly address frequency selective multipath, and MIMO (multiple-input and multiple-output) is more easily implemented when compared with other modulation methods. Specifically, since signal processing is easily performed in a frequency domain using the FFT (Fast Fourier Transform), estimation of a complicated channel matrix and signal separation are easily performed. As an applied technology thereof, multiuser MIMO (MU-MIMO) for communication between a single base station and a plurality of terminals using the same frequency has been discussed.
MIMO is used for communication from a base station, that is, used for a downlink. However, a case where MIMO is used for transmission from a mobile terminal (an uplink) in the future has been discussed. MIMO realizes high-speed communication by transmitting different signals in parallel from different antennas. However, here, it is assumed that all the signals are not delayed from one another and precisely coincide with one another in terms of time. The coincidence is realized by high-accuracy implementation of hardware in a base station. However, when MIMO transmission is performed by a mobile terminal, the precise coincidence in terms of time for individual transmission paths in the terminal is not easily realized. In a mobile terminal, high-accuracy synchronization technique is not employed due to constraint of a size and power consumption. Furthermore, in the MU-MIMO, relative delays of transmissions from terminals make MIMO transmission difficult.
In a case of reception in the OFDM, an FFT frame should be reliably detected. A case where the frame detection fails will be discussed. When FFT is performed before an original start point of a frame, orthogonality of subcarriers after the FFT is maintained if the original start point of the frame is within a guard interval. However, in this case, a guard interval length is deteriorated and desired avoidance of multipath is not performed. On the other hand, if a start timing of the FFT is delayed from the original start point of the frame, intersymbol interference occurs in a tail portion of a signal stream and the orthogonality of subcarriers is not maintained.
Therefore, ingenious synchronization acquisition is applied to a receiver. FIG. 1 is a diagram illustrating a reception apparatus 10 which employs a modulation method using the OFDM and which performs MIMO transmission.
A reception apparatus 10 of FIG. 1 has two reception paths #0 and #1. The reception path #0 has a high-frequency unit (hereinafter referred to as an “RF unit”) 12a connected to an antenna 11a. A signal received by the RF unit 12a is converted into digital data by an analog/digital converter 13a. The data converted by the analog/digital converter 13a is supplied through a matched filter 14a to a correlation detector 15. The matched filter 14a detects a preamble. The correlation detector 15 detects a head position (synchronization point) of an FFT frame using autocorrelation or cross-correlation. In accordance with the head position of the FFT frame detected by the correlation detector 15, a guard interval removal unit 16a removes a guard interval from the received FFT frame.
Data from which the guard interval is removed by the guard interval removal unit 16a is supplied to an FFT unit 17a which extracts data modulated into subcarriers and supplies the extracted reception data of the path #0 to a reception data processor 18.
The reception path #1 has the same configuration as the reception path #0. Specifically, a signal received by an RF unit 12b connected to an antenna 11b is supplied to an analog/digital converter 13b, a matched filter 14b, the correlation detector 15, a guard interval removal unit 16b, and an FFT unit 17b in this order, and reception data of the path #1 is supplied to the reception data processor 18.